fuzzybabybunny:
Say that I want to build a circuit with this LED:
Understanding Ohm's law is useful for this one, plus knowing that an LED operating under usual conditions will have a forward voltage drop of whatever they tell us from the data sheet. Eg 1.8 volt. A resistor in series with the LED will be used to set a desired amount of current that flows through the LED. Maybe 5 mA of current is ok. Depends on how much power the LED can handle.